1. Field of the Inventions
This invention relates to cooling mechanisms and more particularly to an apparatus for cooling high-density integrated circuit packages.
2. DESCRIPTION OF THE PRIOR ART
In the electronics industry in general, and the computer arts in particular, one design objective for some time now has been aimed at increasing operational speeds and decreasing unit sizes. One of the major factors which contributed significantly to this objective is, of course, the integrated circuit chip, or dice as it is sometimes referred to in the industry. Initially, a single dice was mounted in a suitable package, such as a dual-in-line package, and such packages are in common use. However, the trend today is for mounting a plurality of dice in a single package to increase operating speeds by reducing the distances that signals must travel, and also reduce spaces occupied by the individual dice packaging technique.
Significant gains were made when plural dice were mounted on a two-sided substrate and encapsulated in what is known as a JEDEC package. In those packages, electrical connections between the plural dice are made within the encapsulating package, and electrical contacts are provided about the package periphery for connection with with other components of the overall system. The limiting factor of these JEDEC packaging arrangements is the space available on the opposite sides of the substrate for connecting the dice together.
Another relatively recent development in this field is the use of a multi-layer substrate which significantly increases the space available for internal connections. In particular, one package developed by IBM Corporation has successfully mounted 133 dice on a multi-layer ceramic substrate having 33 layers. This package is 90 mm square and has an array of 361 pins depending from the bottom surface of the multi-layer substrate. The pins are soldered into a circuit board which electrically connects the individual pins to the proper components of the overall system.
While this relatively new dice packaging has achieved the desired increased operating speeds, and reduced the overall size, the ultra-high packaging density has created problems relating to heat dissipation. Cooling by radiation into the atmosphere is completely out of the question, and the use of blowers for moving relatively high velocities of air across the packages is inadequate, and liquid cooling systems are being used.
Many of the earlier developed liquid cooling systems, some were developed for use with the JEDEC packages, are inadequate for use with the newly developed multi-layer packaging technology due to insufficient heat transfer between the plural dice and the liquid coolant, and their inability to carry away a sufficient amount of heat generated by the large number of dice.
The cooling system developed by IBM Corporation for use with the hereinbefore described multi-layer packages, includes intricate metal castings, one of which contains the package in a helium filled environment which is provided with 133 bores, each containing a spring-loaded piston. Each of these pistons is in contact with a different one of the dice to carry heat away from the dice through the piston, spring, and metal of the casting. Another metal casting is carried atop the piston casting to provide a chilled coolant chamber, which absorbs heat from the lower casting. This cooling apparatus is an exceptionally complex and costly mechanism, occupies a considerable amount of space, and its thermal transfer efficiency is questionable due to the plural heat conductors and interfacing gaps which are encountered by heat migrating from the dice through the pistons, through the springs, through the metal top wall of the lower casting, through the metal lower wall of the top casting and ultimately to the circulating coolant.
Another prior art structure which is disclosed in my U.S. Pat. No. 4,381,032, includes a base in which the high-density circuit package is nestingly mounted. A heat exchanger is mounted on the base so as to sealingly enclose the circuit package. The heat exchanger includes a rigid housing having a downwardly opening coolant chamber which is enclosed by a thin wall metallic diaphragm that rests in thermally conductive contiguous contact with each of the dice of the integrated circuit package. A liquid coolant is circulatingly moved through the coolant chamber which biases the diaphragm into conductive contact with the dice in addition to its carrying away the heat generated by operation of the integrated circuit package. To insure a more positive contact between the diaphragm and the dice, additional biasing forces are applied to the diaphragm by elastomeric elements, or spring-loaded pistons, provided in the cooling chamber, and these elastomeric elements, or spring-loaded pistons, apply their biasing force to localized areas of the diaphragm with those areas being those which are in contact with the dice of the circuit package. Although this prior art structure is quite efficient, its ability to carry away the operational heat of the operating integrated circuit package is limited by the heat transfer capabilities of the thin-wall diaphragm and the ability of the circulating coolant to absorb the heat transferred thereto by the diaphragm.
In my above referenced related U.S. patents, I filled the coolant chamber with thermally conductive spheres so that they are in heat conductive contact with each other and with the membrane which separates the coolant chamber from the circuit packages. The thermal conductivity of the spheroids is superior to that of the coolant. Therefore, the heat conducted through the membrane wall is readily conducted to the spheroids at each point where they are in contact with the membrane, and the heat is similarly transferred to each of the spheroids in that each spheroid is in contact with the several spheroids adjacent thereto. This results in a substantial increase in the heat exchange surface thereby improving the transfer of heat from the circuit packages to the coolant.
Although filling the coolant chamber with thermally conductive spheroids provides an improvement over prior art cooling systems, there is still some loss in cooling efficiency due to the cooling chamber being separated from the source of heat by the thermally conductive membrane wall.
The existence of florochemical liquid coolants has been known for many years. Such coolants have excellent electrical insulating properties, chemical inertness and low boiling points and are used extensively in cooling large power supplies and other large high voltage devices, with the devices to be cooled being immersed in the coolant. While such coolants are very efficient they have not been widely used due to the heavy weight and cost of the coolant. Also, these florochemical liquids present problems with regard to expansion and contraction and the greater the quantity used in the system to be cooled, the greater are the expansion and contraction problems.
Other special liquid coolants having excellent dielectric properties such as freon, silicone oil, transformer oil, liquid nitrogen and the like are also known. For various reasons, such as those discussed above relating to florochemical liquid coolants, these special liquid coolants have received little or no consideration concerning their use in cooling high density integrated circuit packages.
Therefore, a need exists for a new and improved apparatus for cooling high-density integrated circuit packages which overcomes some of the problems and shortcomings of the prior art.